1. Field of the Invention
This invention relates to the field of sound reproduction, and particularly to apparatus for appreciably reducing distortion produced by non-linear aspects of the driver mechanisms of loudspeakers. More specifically, it relates to significant different and additional distortion reduction made possible by substantial modification of the high-fidelity subwoofer, bass, or lower midrange portion of a loudspeaker of the type which uses at least two almost identical drivers in a push-pull configuration to lower its out-of-phase, even-ordered (2nd, 4th, etc) distortion harmonics very substantially, as will be shown.
Present day feedback systems on loudspeakers (of which several embodiments exist) do not make use of, or distinguish between, in-phase and out-of-phase distortion harmonics. Actually, unless there are (at least) two almost identical drivers, mounted so that each is producing some distortion harmonics out-of-phase with the other, as is precisely the case for the type of push-pull described herein, the question of in-phase or out-of-phase does not even arise.
This push-pull configuration is a prior art concept in which the major even-order distortion harmonics (which contain the 2nd harmonic, usually the largest of all distortion harmonics) are greatly reduced because they are intentionally caused to be precisely out-of-phase as radiated, as between a normally mounted driver (or drivers) and an axially inverted mounted driver (or drivers), as will be explained in detail. What is presented here are some additions and modifications which constitute a specialized, different, and supplemental system capable of providing for the substantial reduction of specifically only the remaining distortion harmonics, a totally different class, all of which are known to be in-phase (as between the two drivers) harmonics and which can not be reduced by the original push-pull concept. From its conception, a new total system was sought which would retain the full operation of push-pull and allow no redundancy or modification of the push-pull system's excellent performance in distortion reduction to occur. This provides a number of important advantages which will be discussed in detail later in this document.
In effect, the invention consists of modifying a push-pull system by using two sensors responsive to motion (commercially available sensors can be suitable as electrical signal sources), one on each cone voice coil assembly of each of the two drivers. Further along the signal paths, a separation of in-phase from out-of-phase (distortion harmonics only), can be made continuously in real time, simultaneously for the many different sounds (and instruments) over the entire bandwidth the device needs, typically 3 or 4 octaves. After removing all the out-of-phase electrical distortion components from the electrical signals, since, remarkably, only the sound components cancelled acoustically out in the air, but the motions of the voice coils, sensors and the electrical signals they produced did not cancel, as will be discussed.
What is left is all the sound fundamentals, all their true undistorted sound harmonics, and all the in-phase distortion harmonics. These can now be fed to an electronic distortion reduction system, using negative feedback, since these are the exact set of signals necessary to reduce all in-phase distortion, maintain all undistorted sound with a moderate drop in gain (easily recoverable by boosting preamplifier gain in advance), and specifically preventing the in-phase system from handling the large, out-of-phase distortion harmonics that push-pull takes care of. Then neither system is spoiled by the presence of the other, and the total result is better than either of them working alone (to be discussed in detail later).
Previous negative feedback systems dealing with distortion harmonics, to the best of the inventors' knowledge, could only lower all distortion harmonics through negative feedback, without selection and therefore without benefit of gain margin relief from another form of distortion reduction as described herein, and obtain the resultant improvement in feedback stability, overload recovery, and high peak transient recovery problems, as well as certain other improvements described later in this document.
One additional advantage of the system to be described here is that it allows separate control of the reduction of two major groups of distortion harmonics, out-of-phase corresponding to purely even-order harmonics over all of the amplitude range and in-phase corresponding to purely odd-order distortion harmonics over almost all of the amplitude range as will be described later in detail. The possible value of separate and independent mechanisms to separate and control the two major groups will also be discussed.
2. Definitions of Terms Used in Prior and Current Art
Some definitions and conventions, as will be used in this description, are defined below.
Push-pull:
refers only and specifically, in this document, to an effective but seldom used prior art method of even-order distortion reduction by mounting in a cabinet one driver (or a group of drivers) in a normal position, that is magnet end in the cabinet, cone facing out, and another driver (or group of drivers) spatially inverted, magnet end out of cabinet, cone facing into the cabinet. (See FIGS. 1a, b, and c.) The two drivers (or groups of drivers) must be driven electrically out-of-phase from each other by a single (or in-phase multiple) power amplifier(s) which cause(s) the drivers to move and radiate all fundamental and true undistorted harmonic sound in-phase and all odd distortion harmonic sound in-phase. The only exception is of one type of very important distortion harmonics, the even-order (including the largest, the 2nd harmonic), produced out-of-phase by one of the few major types of driver non-linearity. For all undistorted sound and all in-phase distortion harmonics, both speakers' voice coils and their cones will move out from the cabinet space at the same time, and into the cabinet space at the same time (in-phase for sound as radiated). Please recall from this paragraph that all of the important acoustic sound waves emitted by both drivers are in-phase despite electrically driving the normal and inverted drivers out-of-phase. Therefore, the drive phase alone is clearly not sufficient cause to produce out-of-phase even-order distortion harmonics. The complete cause will be discussed shortly.
Speaker:
in this document, is generally meant a push-pull subwoofer, bass, or possibly lower midrange portion of a complete audio frequency total spectrum loudspeaker system, unless stated otherwise, and it will be understood to cover a limited frequency range, typically approximately 20 to 125 Hz for a subwoofer, 50 to 200 Hz for a bass, or 150 to 600 Hz for a lower midrange push-pull system.
Subwoofer or bass systems may generally be used occupying a separate, largest volume portion of a full range total spectrum (20 to 20,000 Hz) speaker cabinet including a midrange driver in a separate chamber and an acoustically closed-off, back of the tweeter. Or, most likely (but not necessarily) for a subwoofer, a separate cabinet of its own. FIGS. 1a, b, and c show useful patterns for driver positioning among other useful possibilities with the same principle (not shown). FIG. 1a is seldom used for subwoofers because large diameter drivers are used and the front area becomes too large for acceptable appearance. FIG. 1a, however, could be a good choice for bass or lower midrange. The largest margins for separation of the two drivers are in subwoofers because of the longest wavelength there, so FIGS. 1b and 1c are used, with 1c often preferable because a 1b cabinet, which needs to cleverly disguise the out-of-cabinet driver, may be costly.
Basically, a push-pull speaker consists of a cabinet or portion of a wide range total speaker cabinet, sealed except for circular openings, in which drivers (two or more) are mounted, one normally, the other inverted end-for-end and the drivers are electrically driven out-of-phase. The type system described in this document can also be adapted to vented cabinet systems.
Subwoofer:
is a device for producing audio output down to the order of 20 Hz or lower if required, and up to typically 125 Hz where a normal loudspeaker system can take over or where even quite small satellite speakers are often perfectly appropriate all the way up to 15 or 20 KHz. A subwoofer is often built with an internal amplifier and power supply (called self powered). Among other things, this is because human hearing at very low frequencies such as 30 or 20 Hz requires very high radiated sound power in order to be heard at all, and even more to sound loud. Precise relative levels will be given later. For the moment, consider that an 80 dB sound power level at 1000 Hz (loud) needs 109 dB SPL at 27 Hz to sound just as loud (.about.800 times the sound power level at the ear).
Driver:
is an assembly of a permanent magnet, magnetic flux carrying members forming a relatively uniform field in the gap of a voice coil and a sound radiating cone, with various flexible and rigid support members (see FIG. 2).
Harmonics:
as is commonly used, means a simultaneous production by a voice, instrument, or other sound source of many simultaneous, modified but almost sinusoidal waveforms which therefore necessarily includes harmonics of the fundamental. They come at integral multiples of the frequency of each fundamental sinusoidal waveform (in both electrical and sound form). The first harmonic is the fundamental waveform itself; the second harmonic is a sinusoidal waveform having a frequency of twice the fundamental frequency; the third harmonic, three times the fundamental, etc., and all of these originating in the sound, voice, or instrument being reproduced. The system described in this document handles all of these true sound harmonics as though they were all fundamentals and they are all radiated from the two drivers in-phase.
Distortion harmonics:
Distortion harmonics are at the same frequencies as true, original sound harmonics and occur at an integral multiple of the frequency of any fundamental sound or of any true sound harmonic or distortion harmonic strong enough to cause further (higher frequency) distortion, except that they originate (for the purposes of this document) due only to the deficiencies (non-linearities) of the loudspeaker drivers or other non-linearities of the speaker (or amplifier) system.
In-phase and Out-of-phase distortion harmonics:
defines the phase relationship between a distortion harmonic produced by a normally mounted driver (or group of drivers) and the same frequency distortion harmonic produced by the inverted other driver (or group of drivers) at the same time. Fortunately, the relationship appears to remain fixed as either in or out-of-phase over at least the first 8 (or more) harmonics, and usually after that number amplitudes are too small to be of much consequence.
Odd-order and Even-order distortion harmonics:
Odd-order (3rd, 5th, etc.) distortion harmonics are normally lower in amplitude than the previous (lower numbered) even-ordered distortion harmonics. See FIG. 10 for typical unreduced levels. Odd-orders turn out to be in-phase as between the normally mounted driver and the inverted mounted driver (or group of drivers) and begin to exist and rise in level as the fundamental level rises. Even-order distortion harmonics (2nd, 4th, etc.) begin to exist at even lower fundamental levels and grow as the level rises over the range of weak to very loud listening levels. They are out-of-phase for reasons that will be described in detail later in this document, and are specifically put out-of-phase by having one driver mounted axially inverted and the other mounted normally, i.e. not inverted.
A different and lower amplitude group of even-order distortion harmonics can become of modest significance at higher levels of fundamentals and tend to be quite small themselves (typically 15 to 20 dB down compared to the evens previously mentioned), except in the highest 4 or 5 dB of fundamental level which the drivers are capable of generating. They are in-phase as between inverted and non-inverted drivers and are not caused by non-linearities in the drivers but rather are dependent, on the cabinet's internal volume and the non-linear compression of air in it. They are highly dependent on this volume so a modest increase in cabinet volume can delay their onset and reduce their level.